Keyboard percussion instruments, such as vibraphones and marimbas, are musical instruments that have tone bars and are played upon by musicians with mallets. Keyboard percussion instruments of the type played on by hand-held mallets fall into two distinct categories: non-resonator instruments such as the glockenspiel (orchestra bells) and chimes (tubular bells); and resonated instruments such as the marimba, xylophone and vibraphone (“vibes”).
Resonated instruments such as the vibraphone have resonating air chambers, e.g. resonators, acoustically coupled with associated vibrating tone bars located above. The resonators of keyboard percussion instruments serve to amplify the sound of the tone bar resting above. In order to achieve the optimal relationship between each tone bar and resonator, it is desirable to have the resonator respond sympathetically, or be tuned to, a certain pitch with respect to its associated tone bar. When a resonator is tuned very closely to the pitch of its associated tone bar, the resulting tone when the bar is struck is loud, but relatively short in duration. With a slight amount of de-tuning of the resonator, the resulting tone is not so loud, but persists longer. The degree of de-tuning and whether the de-tuning is above or below the pitch of the tone bar has a significant effect on the quality of the resultant tone. Different musical selections call for different tonal responses. Therefore, it is desirable for the performing artist to be able to select or adjust the response of his instrument to achieve the tonal response for the musical selection to be performed.
Keyboard percussion instruments can be particularly cumbersome in terms of tuning and tone quality. These types of instruments may go out of tune in a variety of environmental conditions. In warm temperature and high humidity, for example, the tone bars may go flat and the resonators may go sharp. In cool and dry weather, the opposite condition may result. These varying conditions have an adverse effect, not only on the pitch of the instrument, but also on the tone quality. Due to the fact that these types of instruments rely on sympathetic resonance of the resonator tube to the tone bar, unmusical results may occur if the two vibrating systems are not in tune.
Despite these problems with varying environmental conditions as well as tuning, keyboard percussion instruments are usually sold with non-movable force-fit metal stops in the resonator tubes. A forced-fit, domed metal stop produces a strong, long-ringing (lossless), brightly toned, reflective surface. The resonator, usually made from aluminum or brass tubing, has no air leakage or energy losses because the metal plug is forced into the tube (e.g., with a hydraulic press), producing a perfect air seal around the circumference between the outer diameter surface of the stop and the inner wall of the resonator tube.
Many instruments of this type are supplied with one end of the resonator tube (the end furthest from the tone bar), permanently sealed at a distance that produces a resonant frequency equal to the associated tone bar above. When the resonator is associated with a tone bar tuned to, for example, A=440, the manufacturer sets the stop in the tube to produce a resonant frequency as close to A=440 as possible. Positioning the stop slightly sharp or flat to A=440 produces different results, such as altered ring times, volume and timbre. These permanent stops are prepositioned at the factory to resonate the above-suspended bar at a particular temperature and humidity level.
The position of these plugs is determined not only by the temperature and weather conditions at the point of manufacture but also by the taste of the designer and accidents and/or inconsistencies of manufacturing. When the instrument is played in an environment that exactly duplicates that for which it was tuned (e.g., about 50% humidity and 72° F.), these resonators should perform well. A reduction of the ambient air temperature by as little as 4° F., however, substantially reduces the volume potential of the instrument while increasing the apparent ring-time of the bar, adversely influencing the tone character of the combined bar/resonator system. Conversely, an increase of 4° F. in the ambient temperature reduces the apparent ring-time of the bar/resonator system to a level that even a lay person can hear easily.
Until recently, musicians have generally had to endure these shortcomings in performance. Even if the musician could take along all the wood-working or metal-working equipment to tune the tone bars at the performance site, this would not be a viable method to compensate for transitory weather conditions because tuning the tone bar requires removing material from the bar. After a few tunings by removing material, permanent loss of mass begins to be audible as loss of tone quality. Thus, a better way to bring these two sympathetically-vibrating systems into musical resonance is to change the effective length of the resonator tube.
Several movable/tunable stop systems have been introduced. Although some of them offer tuning and volume advantages over the permanently installed metal stops used by the vast majority of manufacturers, each has one or more significant drawbacks. For instance, some versions of movable stops merely squeeze a flexible rubber membrane between two rigid plastic disks or plates. While this system is functional and produces an air-tight seal, it does not produce good musical or ergonomic results. One drawback with this type of stop is that it is very slow to tighten and/or loosen the clamping device sufficiently to allow movement. Additionally, the assembly can easily camber out of 90-degrees square to the inside wall of the tube as it is being loosened or adjusted. It can also be incorrectly tightened in an out-of-square, cambered position. Any of the out of square cambered conditions produces unacceptable “out of focus” timbre and false harmonics. Also, the system is never completely rigid after tightening the two plates together, because the rubber membrane sealing against the inside diameter of the tube is by soft and flexible. Lastly, a significant cross-section of the soft, flexible material is exposed to the vibrating air column in the resonator.
Other versions of movable/tunable stop systems use an O-ring around a disk. While this system is functional and produces an air-tight seal, it does not produce good musical or ergonomic results. When the stop is moved against the friction of the O-ring, the O-ring alternately drags and then rolls, so that only certain positions of the stop can be selected. Additionally, the cleanliness or “focus” of the pitch and harmonicity of the overtones are degraded by the pitch conflict produced by the space between the flat top plate and the actual seal, which is a bit further from the open end of the tube. Other hybrid designs, such as squeezing the O-ring between two disks or plates also share many of these same shortcomings.